Germanium (Ge) has a high density and is relatively hard with excellent transmission features of blocking UV but transmitting infrared from 2µm-14µm. Germanium has low dispersion and the highest refractive index of most commonly used IR-transmitters in these wavebands so desirable for IR lens design where its high refractive index allows otherwise impossible specifications to be met.
Germanium an excellent choice for windows in IR as it transmits over 45% between 2-14µm up to 45°C, while transmission starts to drop at 100°C. BBAR coatings are advisable as use of Antireflective coating increases transmission to over 80% between 3µm-14µm. For harsh environments a diamond like coating (DLC) on one face is advisable. Germanium is unsuitable for high temperature applications.
Germanium windows and lenses are typically used in thermal imaging where it’s used as a front protective usable optics. Its index of refraction makes Germanium useful for wide-angle lenses. Germanium is also used for FTIR (Fourier Transformed Infrared) and FLIR (Forward Looking Infrared) and spectroscopy systems.